To your question:
Q: Can anyone explain to me the difference between a Junior Associate & an Associate role at McKinsey?
Expectations are pretty similar. As Junior Associate you would have more support in your initial projects until you prove you can lead a workstream totally independent.
Q: Also, in the Expert track, for an experienced hire, I believe he/she would join as a Junior Specialist. Is that equivalent to a Junior Associate role in the Generalist track?
It is. Career speed and salaries are the same. Only differences are in career paths. As expert, you choose between being fully expert or Expert EM at some point and if you on the second, you can only reach to Expert Partner (specialist cannot become Senior Partners)